The yellow arrow points to the top lip of the magazine adapter. The magazine adapters look a little different depending on the caliber of the ammunition used in the gun, but they are all removed the same way. Note that the upper edge of the adapter is just below the circular projection in the magwell. 

Removal of the mag adapter from a CX4 is quite simple-- more simple than these instructions will make it sound! 

First, split the gun. Then, looking down into the magwell, find the top lip of the magwell adapter. This will be on the rear face of the magwell about one inch below the hammer pivot spring (see the photo above). Reach down to the top lip of the magazine adapter with your finger and pull the top lip of the adapter forward, away from the rear wall of the magwell. At the same time as you pull forward, apply downward pressure on the adapter. If you pull far enough the hook on the back of the adapter will disengage and the adapter will slide out of the bottom of the magwell. It may help to use a finger of your sapre hand to reach inside the adapter from below to provide additional downward force on it.

To re-install the adapter simply push it up into position until you hear the click as the hook on the back of the adapter engages in its recess.